Thinking About Returning Myself...
T-Mobile is bad where I live too. I had to drop them when suddenly I wasn't getting service at my house and they couldn't tell me why. They let me out of my contract and I went with Sprint. I hope AT&T is as good as Sprint because I'm headed their way if Sprint continues to ignore WP.
I had AT&T since 1998 and switched to Sprint a little over 2 years ago for the unlimited data/any mobile to any mobile. I only had a couple of issues with AT&T's billing policies, but those were fairly quick to resolve. They do seem to have understanding CS reps that are very willing to help. Their call quality was on par with Sprint in that there was an occasional dropped call, but few and very far between. Reception in the Sacramento area was good as is with Sprint.
I'm with you, GG916, in that we are right on the verge of switching back to AT&T, also. No real complaints with Sprint, either, except for their lack of WP7 love. I'm out of contract now and waited patiently to see if they would announce any WP7 devices at the CES last week. Unfortunately, that was not to be - and I'm not sure I want to wait the 8 or 9 months only to be disappointed again.
I compared prices and with my employer's discount and Sprint's modified employer's discount on the family plan, the price is almost identical. Sure we'll have a 2GB limit per phone, but with WiFi I think we'll be fine with any increased usage. Since I have had Sprint, we have never went over the 2GB mark between all 4 phones on the plan, anyways. I'll wait until March, probably, to see (1) what the Lumia 900 price is and (2) if the Focus S price drops with the introduction of the Nokias.
I really don't see Sprint getting any WP7 phones in in the near future. And I can foresee them going to a tiered plan when they roll out the LTE. Unfortunately, for those still with Sprint, I doubt that they will grandfather the unlimited data after their current contracts run out. So, now is a good a time as any to make the switch.